1. Field of the Invention
The invention relates generally to three dimensional circuit arrays.
2. Background
Typical circuit configurations include a plurality (sometimes hundred, thousands, or millions) of devices formed on a substrate such as a semiconductor substrate or chip and interconnected perhaps to one another or to circuitry external to the substrate through interconnect layers (as many as five or more) over the substrate. Active devices in this case includes, but are not limited to, transistors, capacitors, resistors, diodes, and programmable media (collectively xe2x80x9cactive mediaxe2x80x9d).
Typical memory applications include dynamic random access memory (DRAM), static random access memory (SRAM), erasable programmable read only memory (EPROM), and electrically erasable programmable read only memory (EEPROM). Solid state memory devices typically employ micro-electronic circuit elements for each memory bit (e.g., one to four transistors per bit) in memory applications. Since one or more electronic circuit elements are required for each memory bit, these devices may consume considerable chip xe2x80x9creal estatexe2x80x9d to store a bit of information, which limits the density of a memory chip.
State or phase change memory devices use materials that can be electrically switched between a generally amorphous and a generally crystalline state. One type of memory element developed by Energy Conversion Devices, Inc. of Troy, Mich. utilizes a phase change material that can be, in one application, electrically switched between a structural state of generally amorphous and generally crystalline local order or between different detectable states of local order across the entire spectrum between completely amorphous and completely crystalline states. Typical materials suitable for such application include those utilizing various chalcogenide elements. These electrical memory devices typically do not require field effect transistor select devices, and comprise, in the electrical context, a monolithic body of thin film chalcogenide material. As a result, very little chip real estate is required to store a bit of information, thereby providing for inherently high density memory chips. The state change materials are also truly non-volatile in that, when set in either a crystalline, semi-crystalline, amorphous, or semi-amorphous state representing a resistance value, that value is retained as that value represents a physical state of the material (e.g., crystalline or amorphous). Thus, phase change memory materials represent a significant improvement in non-volatile memory.
One integrated circuit advancement effort is to increase the number of devices (active media) that can be located on a substrate (e.g., chip). In terms of memory device applications, for example, although phase change memory devices (typically arrays of memory devices) occupy significantly less real estate than traditional solid state memory devices, there remains a desire to increase the device (active media) density.